This invention relates in general to the field of rapidly energizing coils and in particular to controlling stepping motors with a low-power motor drive circuit.
Many electronic applications involving solenoids, transformers, and other inductors involve energizing an inductor which has a relatively long characteristic time constant. One such electronic device containing an inductive coil is a stepper motor. Stepper motors are used extensively in electromechanical positioning systems, with applications in tape drives, floppy disk drives, numerically controlled machinery, and other digitally controlled positioning systems. Stepper motors are particularly important for electronic products such as printers, scanners, plotters, and facsimile machines.
Stepper motor applications often require high step rates, which can be achieved by increasing the size of the stepper motor (at increased cost), or by using a smaller motor (with less torque) and overdriving it to increase its speed. Long characteristic time constants relative to short desired response times can cause poor or unacceptable performance in the stepper motor within a printer, scanner, or plotter.
Rapid stepping requires a high rate of change of current over time in the stepper motor windings. Current overvoltage stepper motor drive circuits attempt to correct poor stepper motor response by driving the stepper motor with a higher voltage than the nominal voltage. For example, an R-4R technique can be used to drive a 5 Volt (V) stepper motor of internal resistance R with 25 V across a 4R resistor in series with the stepper motor. The result is that the current in the stepper motor rises faster with the fivefold increase in voltage than it would otherwise. The cost of the technique, however, is that the 4R resistor continues to dissipate power after the stepper motor has been fully energized. In fact, a full 80% of the energy supplied is dissipated by the 4R resistor, while the motor receives only 20% of the input energy. Thus, the efficiency of such a design is 20% at maximum.
An improvement on the R-4R circuit can also be employed. Adding a sense amplifier on a control transistor in the high voltage supply (25 V in the last example), can be used to achieve a faster response. The improvement still dissipates 80% of the power supplied by the series limit circuit, however.